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The low-x data of the structure function F2 measured by the collaborations H1 and ZEUS are compared with the prediction of the DGLAP equations. A new method of comparing data and theory is presented. The systematic and quantitative analysis shows that the predicted 1/x behavior deviates increasingly from the observed one as x decreases below 10-3.

The strong coupling constant αs is determined from inclusive jet and dijet cross sections in neutral-current deep-inelastic ep scattering (DIS) measured at HERA by the H1 collaboration using next-to-next-to-leading order (NNLO) QCD predictions. The dependence of the NNLO predictions and of the resulting value of αs(mZ) at the Z-boson mass mZ are studied as a function of the choice of the renormalisation and factorisation scales. Using inclusive jet and dijet data together, the strong coupling constant is determined to be αs(mZ)=0.1157(20)exp(29)th. Complementary, αs(mZ) is determined together with parton distribution functions of the proton (PDFs) from jet and inclusive DIS data measured by the H1 experiment. The value αs(mZ)=0.1142(28)tot obtained is consistent with the determination from jet data alone. The impact of the jet data on the PDFs is studied. The running of the strong coupling is tested at different values of the renormalisation scale and the results are found to be in agreement with expectations.

Cross sections for elastic and proton-dissociative photoproduction of J / ψ mesons are measured with the H1 detector in positron-proton collisions at HERA. The data were collected at ep centre-of-mass energies and , corresponding to integrated luminosities of and , respectively. The cross sections are measured as a function of the photon-proton centre-of-mass energy in the range 25< W γp <110 GeV. Differential cross sections d σ /d t , where t is the squared four-momentum transfer at the proton vertex, are measured in the range | t |<1.2 GeV 2 for the elastic process and | t |<8 GeV 2 for proton dissociation. The results are compared to other measurements. The W γp and t -dependences are parametrised using phenomenological fits.

The HERAPDF2.0 ensemble of parton distribution functions (PDFs) was introduced in 2015. The final stage is presented, a next-to-next-to-leading-order (NNLO) analysis of the HERA data on inclusive deep inelastic ep scattering together with jet data as published by the H1 and ZEUS collaborations. A perturbative QCD fit, simultaneously of αs(MZ2) and the PDFs, was performed with the result αs(MZ2)=0.1156±0.0011(exp)-0.0002+0.0001(model+parameterisation)±0.0029(scale). The PDF sets of HERAPDF2.0Jets NNLO were determined with separate fits using two fixed values of αs(MZ2), αs(MZ2)=0.1155 and 0.118, since the latter value was already chosen for the published HERAPDF2.0 NNLO analysis based on HERA inclusive DIS data only. The different sets of PDFs are presented, evaluated and compared. The consistency of the PDFs determined with and without the jet data demonstrates the consistency of HERA inclusive and jet-production cross-section data. The inclusion of the jet data reduced the uncertainty on the gluon PDF. Predictions based on the PDFs of HERAPDF2.0Jets NNLO give an excellent description of the jet-production data used as input.

The H1 Collaboration reports the first measurement of the 1-jettiness event shape observable τ 1 b in neutral-current deep-inelastic electron-proton scattering (DIS). The observable τ 1 b is equivalent to a thrust observable defined in the Breit frame. The data sample was collected at the HERA ep collider in the years 2003–2007 with center-of-mass energy of s = 319 GeV , corresponding to an integrated luminosity of 351.1  pb - 1 . Triple differential cross sections are provided as a function of τ 1 b , event virtuality Q 2 , and inelasticity y , in the kinematic region Q 2 > 150 GeV 2 . Single differential cross sections are provided as a function of τ 1 b in a limited kinematic range. Double differential cross sections are measured, in contrast, integrated over τ 1 b and represent the inclusive neutral-current DIS cross section measured as a function of Q 2 and y . The data are compared to a variety of predictions and include long-standing and more recent Monte Carlo event generators, predictions in fixed-order perturbative QCD where calculations up to O ( α s 3 ) are available for τ 1 b or inclusive DIS, and resummed predictions at next-to-leading logarithmic accuracy matched to fixed order predictions at O ( α s 2 ) . These comparisons reveal sensitivity of the 1-jettiness observable to QCD parton shower and resummation effects, as well as the modeling of hadronization and fragmentation. Within their range of validity, the fixed-order predictions provide a good description of the data. Monte Carlo event generators are predictive over the full measured range and hence their underlying models and parameters can be constrained by comparing to the presented data.

The Breit frame provides a natural frame to analyze lepton–proton scattering events. In this reference frame, the parton model hard interactions between a quark and an exchanged boson defines the coordinate system such that the struck quark is back-scattered along the virtual photon momentum direction. In Quantum Chromodynamics (QCD), higher order perturbative or non-perturbative effects can change this picture drastically. As Bjorken- x decreases below one half, a rather peculiar event signature is predicted with increasing probability, where no radiation is present in one of the two Breit-frame hemispheres and all emissions are to be found in the other hemisphere. At higher orders in α s or in the presence of soft QCD effects, predictions of the rate of these events are far from trivial, and that motivates measurements with real data. We report on the first observation of the empty current hemisphere events in electron–proton collisions at the HERA collider using data recorded with the H1 detector at a center-of-mass energy of 319 GeV. The fraction of inclusive neutral-current DIS events with an empty hemisphere is found to be 0.0112 ± 3.9 % stat ± 4.5 % syst ± 1.6 % mod in the selected kinematic region of 150 < Q 2 < 1500 GeV 2 and inelasticity 0.14 < y < 0.7 . The data sample corresponds to an integrated luminosity of 351.1 pb - 1 , sufficient to enable differential cross section measurements of these events. The results show an enhanced discriminating power at lower Bjorken- x among different Monte Carlo event generator predictions.

Inclusive e(+/-)p single and double differential cross sections for neutral and charged current deep inelastic scattering processes are measured with the H1 detector at HERA. The data were taken at a centre-of-mass energy of root s = 319 GeV with a total integrated luminosity of 333.7 pb(-1) shared between two lepton beam charges and two longitudinal lepton polarisation modes. The differential cross sections are measured in the range of negative four-momentum transfer squared, Q(2), between 60 and 50 000GeV(2), and Bjorken x between 0.0008 and 0.65. The measurements are combined with earlier published unpolarised H1 data to improve statistical precision and used to determine the structure function xF(3)(gamma Z). A measurement of the neutral current parity violating structure function F-2(gamma Z) is presented for the first time. The polarisation dependence of the charged current total cross section is also measured. The new measurements are well described by a next-to-leading order QCD fit based on all published H1 inclusive cross section data which are used to extract the parton distribution functions of the proton.